1. Field of the Invention
The present invention relates to a flow monitoring device and more particularly to a flow monitoring device including a heated sensor which forms part of a bridge circuit that furnishes electrical heating power to the sensor to maintain the temperature of the sensor above the ambient temperature at a reference sensor upstream from the heated sensor. When the heated sensor is exposed to a flow stream of liquid, the flow stream carries heat away from the heated sensor and the rate at which heat is lost from the sensor is a direct measurement of liquid velocity. Such a flow monitoring device further includes electrical control circuitry for maintaining a predetermined temperature differential between the upstream sensor and the heated sensor and for sensing the amount of electrical/heat energy required to maintain the temperature differential which is indicative of the change of the flow rate of the liquid above or below a predetermined flow rate.
2. Description of the Prior Art
Heretofore flow monitoring devices of the type which include a heated sensor and an upstream reference sensor have been proposed for sensing the flow rate of liquid such as biomedical fluids.
For example, a flow monitoring sensor is disclosed in the fall, 1980 NASA Tech Briefs in an article entitled: "Flow Sensor for Biomedical Fluids". This publication discloses the provision of upstream and downstream thermistors which have a constant temperature difference maintained between them by a resistance heater. The electric power consumed by the heater in maintaining the constant temperature difference is a measure of the flow rate which is used to control accurately the flow rate through a pump or valve.
In French Pat. No. 2 278 061 a fluid flow measurement device that detects differential heating in a pipe using a bridge circuit with two thermistor arms is disclosed.
In Bulletin 600 published by CGS/Datametrics, a division of CGS Scientific Corporation of Watertown, Mass. entitled "Heat Sensor Finds Wide Applications in Fluid Flow Measurements" there is disclosed a heated sensor that is part of a bridge circuit that furnishes electric heating power to the sensor to maintain the temperature of the sensor above the ambient temperature upstream of the liquid flow.
Similarly, a flow meter operating on a temperature difference between sensors that are situated in legs of a bridge circuit is disclosed in German printed patent application no. 27 35 118.
Other systems for monitoring a flowing liquid are disclosed in U.S. Pat. Nos. 3,279,251; 3,535,927; and 3,898,637.
As will be described in greater detail hereinafter, the flow monitoring device of the present invention and the housing assembly forming a part thereof differ from the previously proposed flow monitoring systems disclosed in the publications referred to above by providing means for enhancing the sensing of the temperature of the flowing liquid at the upstream end and at the heated sensor. Such enhancement of temperature sensing is accomplished by providing enlarged cross section sensing chambers in the housing assembly where the flow of the liquid is slowed to enhance temperature sensing and which chambers are configured to minimize turbulent flow through the chambers.
In a preferred embodiment, the housing assembly has transverse bores which extend from an outer side wall of the housing assembly to respective ones of the sensing chambers. A thermally conductive thimble shaped member is received in and through each one of the transverse bores in a fluid tight manner and extends into the respective sensing chamber. This construction enables the housing assembly to be removably mounted over and on a heated sensor and a reference sensor and then removed therefrom and discarded with a tubing liquid delivery set. This construction also provides non-invasive monitoring of the flow of liquid through the housing assembly of the flow monitoring device.
Furthermore, the flow monitoring device has a reduced in cross-section passageway communicating between the larger in cross-section chambers, which reduced in cross-section passageway serves to "isolate" the chambers from each other. In other words, the higher velocity flow through the passageway prevents "thermal pollution" between the chambers.
This is important since, heretofore, with prior devices utilizing a cylindrical chamber containing both the reference and heat loss probes, effective performance at medium and low flow rates required vertical orientation of the cylindrical chamber with the heat loss probe at the top. With that orientation, "thermal pollution" due to convection flow and differences was prevented. With the heat loss probe at the bottom, the prior device was ineffectual and with horizontal orientation rate change detection sensitivity was reduced.
In contrast, the flow monitoring device of the present invention with the reduced-in-cross-section connecting passageways can be effectively operated in any position.
Such housing assembly further differs from the previously proposed flow monitoring systems by its construction from two identical end parts and one middle part which are easily molded from a clear plastic material and easily assembled together to form a passage means therethrough with spaced apart larger cross section sensing chambers.